Image sensor arrays typically comprise a linear array of photosensors which raster scan an image bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors.
For high-performance image sensor arrays, a preferred design includes an array of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging without reductive optics. In order to provide such a “full-width” array, relatively large silicon structures must be used to define the large number of photosensors. A preferred technique to create such a large array is to make the array out of several butted silicon chips. In one proposed design, an array is intended to be made of 20 silicon chips, butted end-to-end, each chip having 248 active photosensors spaced at 400 photosensors per inch.
Although most scanning systems currently in use are ultimately digital systems, the “raw signal” coming out of the photosensors during the scanning process is an analog video signal, with the voltage magnitude corresponding to the intensity of light impinging on the photosensor at a given time. Thus, when signals are readout from the photosensors on a chip to be converted to digital data, different video levels, corresponding to the brightness of the reflected area being scanned by a particular photosensor at a particular moment, are output as a series of analog voltage levels.
Photosensitive devices may be one-dimensional or two-dimensional, and can be either of the “active” variety, wherein the photosensors output voltage signals, or in the form of a charge-coupled device, or CCD, which outputs a sequence of charges from a series of individual photosensors. In all of these various types of photosensitive devices, a common design feature is the use of “dark” photosensors, which are used to periodically reset the offset voltage for the photosensors being read out. These dark photosensors are of the same semiconductor structure as the other “active” photosensors on each chip, but the dark photosensors are not exposed to light. In most designs, the dark photosensors are provided with an opaque shield, such as of aluminum or silicon, to prevent the influence of light thereon. In the scanning process, with each readout cycle of active photosensors on each chip, the readout of the first photosensor is proceeded by readouts of one or more dark photosensors, which are used to reset the voltage offset associated with the whole chip, and thereby correct signal drift when the active photosensors are reading out their signals. In other words, the readout of a dark photosensor with each scan can serve as a reference offset or “zero point” so that the absolute values of light intensity on the active photosensors may be determined. The use of a dark photosensor output when reading out signals from active photosensors can significantly compensate for performance variations of multiple chips in a single apparatus, and also for changes in the performance of a photosensitive device over time.
With any sophisticated system for reading out images signals from a series of photosensors, a common practical problem is known as “fixed-pattern noise.” With each individual photosensor for an associated transfer circuit, there is likely to be a single dedicated amplifier. Given the practicalities of constructing photosensors and circuits on a chip, it is likely that certain amplifiers, associated with certain photosensors, will consistently have higher output relative to other amplifiers associated with other photosensors. There exist basic techniques for overcoming fixed pattern noise, such as mentioned in U.S. Pat. No. 5,654,755, which will be described in detail below.
In order to increase the readout speed of image signals from, for example, a linear array of photosensors, it is known to provide a separate “odd” and “even” channels for the output of image signals. A basic example of this technique is shown in U.S. Pat. No. 5,638,121. In brief, odd and even photosensors along a linear array respectively output image signals into separate video lines, and these video lines are subsequently multiplexed downstream, thus yielding a single video stream representative of both odd and even video lines. As a practical matter, it has been found that such a design may exhibit a “video path offset” distortion effect caused by the fact that, where the odd and even video lines are separate, the odd and even video paths each go through a different set of amplifiers and are thus consistently given slightly different levels of amplification.
The present invention is directed toward a readout system for video signals in which multiple video lines are multiplexed, in a matter which eliminates both fixed pattern of noise and video path offset.